Since the recent discovery of electroluminescence (EL) in both small organic molecules and conjugated polymers, organic and polymeric light-emitting diodes (OLEDs and PLEDs) have been a focus of organic electronics. In order for these devices to be integrated into displays, it is required to pattern the light-emitting materials into small, multi-layered elements for full-color visualization. While OLEDs rely on vapor deposition of small molecules, PLEDs can access a wider range of options if suitable processing methods are available. Solutions of conjugated polymers can be dispensed onto the desired area by ink jet printing or screen printing, or form films on regions where a sacrificial photoresist material defines the target. Although the photolithographic methods are in principle more efficient and possess higher resolution, they have not been recognized as suitable for PLEDs. There has been concern that organic solvents used in photoresist deposition and stripping harm the integrity of the active organic materials. This challenge has motivated the identification of new lithographic processes where less damaging solvents are employed.
Solution processing of organic electronic materials is a highly attractive processing option for many applications, particularly organic light emitting diodes (OLEDs) for display and solid-state lighting. It is a low cost approach with no limitations with regard to substrate size. While highly efficient full color displays are rather straightforward to fabricate via vacuum-assisted shadow mask deposition of organic small molecules, it is challenging to achieve solution-processed full color displays due to the limitations imposed by compatibility issues among active light-emitting components and other chemicals and solvents used in the device patterning process.
Much work has been done on the patterning of organic electronic materials, However, patterning techniques such as inkjet printing and screen printing suffer from the disadvantages of low resolution and low throughput. As such, photolithography is still the ideal technique for patterning of organic light-emitting materials, since it has good resolution, high-throughput, easy scalability to large substrates, good registration between multiple layers and is very well established in the semiconductor industry. However, the standard organic and polar solvents used in the processing of photosensitive materials can damage the organic light-emitting materials used as active layers. Several approaches have been proposed to overcome this problem. For example, light-emitting polymers with side-groups can be cross-linked under light activation to produce insoluble polymer networks in desired areas. Inserting a photocurable interlayer between the active layer and the substrate and simultaneously patterning both layers can be an alternative less damaging approach. Depositing a buffer layer (parylene-C and CYTOP, respectively) underneath the photoresist film was an effective way to protect the underlying active organic films during the photolithographic processing steps. Also, fluorinated imaging material can be used in combination with fluorous solvents to pattern a wide variety of non-fluorinated organic electronic materials, including poly(9,9-dioctylfluorene) (F8) and poly(3-hexylthiophene) (P3HT) without causing device damage. Another alternative approach to pattern polymer light-emitting diodes (PLEDs) uses dry photolithography (DPP) via a supercritical CO2 process.
The cross-linkable light-emitting polymers mentioned above also provide an approach to achieve solution-processed multi-layer OLED structures. Nevertheless, this option involves complicated chemical synthesis and requires careful polymer handling to prevent undesired cross-linking of the polymer. Moreover, curing agents are unnecessary components for device operation and even generate a significant amount of chemical residue that remains a challenge to device lifetime. Materials orthogonality has been utilized to form three-layer solution-processed light-emitting devices by alternate deposition of hydrophobic polymer and hydrophilic polyelectrolyte solutions. However, the significant amount of mobile ions carried by the polyelectrolytes is known to limit the operating lifetime. In addition, the polar solvents used for dissolving electrolytes, usually water or alcohols that are known to be detrimental to device carrier mobility and lifetime, thus they should be avoided during device processing.
Based on the foregoing, there exists an ongoing and unmet need for methods and materials for patterning of organic materials.